Silicon wafer for IGBT and method for producing same

ABSTRACT

A silicon wafer for an IGBT is produced by forming an ingot having an interstitial oxygen concentration [Oi] of not more than 7.0×10 17  atoms/cm 3  by the Czochralski method; doping phosphorus in the ingot by neutron beam irradiation to the ingot; slicing a wafer from the ingot; performing annealing of the wafer in an oxidizing atmosphere containing at least oxygen at a temperature satisfying a predetermined formula; and forming a polysilicon layer or a strained layer on one side of the wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon wafer which is suitably usedas a substrate for an insulated gate bipolar transistor (IGBT) and amethod for producing the same. Specifically, the present inventionrelates to a silicon wafer for IGBT, which is produced through aCzochralski method (CZ method).

Priority is claimed on Japanese Patent Application No. 2005-169929,filed on Jun. 9, 2005, the content of which is incorporated herein byreference.

2. Description of Related Art

An insulated gate bipolar transistor (IGBT) comprises a MOSFET (MetalOxide Semiconductor Field Effect Transistor) provided with a PN junctionfor hole injection. In the IGBT, a gate and an emitter are formed on asurface of an n⁻ type silicon layer having high resistitvity, and acollector is formed on the backside of the n⁻ type silicon layer via aPN junction. An IGBT is an element in which an electric current betweenthe emitter and the collector is controlled by an electric voltageapplied to gates intervening a silicon oxide film. In an IGBT, it ispossible to reduce the on resistance by injection of hole from thecollector to an n⁻ type silicon wafer positioned between the gate,emitter and collector. In addition, an IGBT is not easily destroyed evenafter a high electric current flows therethrough.

As described above, in the IGBT, an electric current is controlled bythe gates intervening an oxide film. Therefore, it is desirable that thegate oxide film be free of defects. In addition, since the current flowsbetween the emitter on the surface of the element and the collector onthe back side of the element, defects formed on the wafer have a largeinfluence on the property of the IGBT. Therefore, in the prior art,epitaxial wafers or silicon wafers obtained from a crystal formed by theFZ method (Floating-Zone Melting Method) have been used as a siliconlayer for an IGBT.

However, in order to constitute an IGBT of high dielectric resistance,an n⁻ type silicon layer is required to have a thickness of about 100μm. In order to realize such a thickness by epitaxial growth of thelayer, a long production time is required, and therefore an increase inproduction cost cannot be avoided. When a crystal (FZ crystal) isproduced by the FZ method, the amount of impurities contaminating thecrystal during a production process of the crystal is smaller than thatof a crystal (CZ crystal) produced by a CZ method. Therefore, it ispossible to obtain a relatively defect-less silicon wafer by slicing thewafer (FZ wafer) from an FZ crystal, compared to the case in which asilicon wafer (CZ wafer) is sliced from a CZ crystal. On the other hand,it is difficult to produce a large crystal by the FZ method. Therefore,the FZ method is not appropriate for wafers having large diameter, ormass production of wafers.

On the other hand, silicon wafers sliced from a crystal produced by theCZ method include defects comprising microscopic voids of 0.1 to 0.3 μmin size. If such defects are exposed to a surface of a wafer, thedefects form pits on the wafer surface. Those defects are generallycalled as COPs (Crystal Originated Particles). It has been impossible touse a silicon wafer having COPs as it is as a wafer for IGBT. Recently,as described in PCT international publication No. WO 2004/073057 (PatentDocument 1), a method for producing a wafer has been developed, in whichthe numbers of COPs in the wafer have been reduced by performing a heattreatment of a wafer obtained through the CZ method.

Since a large sized crystal can be grown by the CZ method, the CZ methodcan easily provide a wafer having a large diameter. For example, massproduction of wafers having a diameter of 300 mm has been realized.Therefore, a CZ wafer (a wafer sliced from a crystal grown by the CZmethod) is suitable as a wafer for large scale integration circuit(LSI). However, CZ wafers have not been used as substrates for IGBTbecause of the following problems.

Firstly, there is a problem in terms of the yield of GOI (Gate OxideIntegrity). During the growth of a single crystal in the CZ method,excess vacancies occasionally condense to generate void defects of 0.2to 0.3 μm in size. As described-above, those defects are called COPs.Surface pits formed by the exposure of COPs on the wafer surface or COPsin the vicinity of the wafer surface may be captured in the oxide filmlayer during a thermal oxidation process causing deterioration of GOIproperty. Therefore, in order to inhibit the influence on the GOIproperty, it is necessary to eliminate COPs.

Secondly, there is a problem in terms of fluctuation of resistivity. Asilicon single crystal grown by the CZ method (CZ silicon) includesexcess oxygen in the order of 1×10¹⁸ atoms/cm³. Therefore, by a lowtemperature heat treatment at a temperature of about 450° C., oxygendonors are caused to occur and fluctuate the resistivity of a substrate.Therefore, it is important to inhibit the occurrence of oxygen donors.

Thirdly, there is a problem in terms of homogeneity in resistivity.Resistivity of a CZ silicon can be controlled by the amount of dopantadded to the polycrystalline silicon. However, because of the smallsegregation coefficient of phosphorus (P), which is an element used inthe substrate for an IGBT, concentration of phosphorus varies greatly inthe lengthwise direction of a single crystal ingot. Therefore, wafershaving a specific resistivity can be obtained only from a narrow portionof a single crystal ingot.

Fourthly, there is a problem in terms of deterioration of therecombination lifetime. As described above, a CZ silicon generallyincludes oxygen in an order of 1×10¹⁸ atoms/cm³. Therefore, during athermal process accompanied by a device formation process, excess oxygenprecipitates to form SiO₂ deteriorating the recombination lifetime.

The technology disclosed in Patent Document 1 enables elimination ofCOPs, which are the factors deteriorating the GOI property. Therefore,the technology is applicable as a method for producing silicon wafersfor IGBT. However, the technology described in Patent Document 1 has thefollowing problem. Although the oxygen concentration can be controlledto a level allowing the elimination of COPs, the oxygen concentrationmay be sufficiently high to cause the above-described generation ofoxygen donors, resulting in fluctuation of the resistivity orprecipitation of excess oxygen resulting in the deterioration of therecombination lifetime. Especially, the deterioration of therecombination lifetime is highly undesirable in a silicon wafer forIGBT. Therefore, in order to use a silicon wafer as a silicon wafer foran IGBT, it is necessary to reliably inhibit the deterioration of therecombination lifetime, which may be caused by the oxide precipitation,as well as by contamination of heavy metals.

Based on the above-described consideration, an object of the inventionis to provide a silicon wafer which is obtained from a silicon ingotgrown by the CZ method and is suitably applicable for an IGBT, and amethod for producing such a silicon wafer.

SUMMARY OF THE INVENTION

In the invention, the following constitution was adopted for achievingthe above-described object.

A method for producing a silicon wafer for an IGBT of the inventioncomprises:

by the Czochralski method (CZ method), performing growth of a siliconingot having an interstitial oxygen concentration [Oi] of not more than7.0×10¹⁷ atoms/cm³;

performing doping of phosphorus in the silicon ingot by performingneutron beam irradiation to the ingot;

slicing a wafer from the ingot;

performing annealing of the wafer in an oxidizing atmosphere containingat least oxygen at a temperature of T(° C.) that satisfies a formula,[Oi]≦2.123×10²¹ exp(−1.035/k(T+273))  (1); and

forming a polysilicon layer or a strained layer on one side of thewafer,

wherein [Oi] is measured by Fourier transform infrared spectroscopybased on a standard of ASTM F-121(1979); and k is Boltzman constant(8.617×10⁻⁵(eV/K)).

In the above-described method for producing a silicon wafer for an IGBT,it is preferable to form a silicon ingot having a nitrogen concentrationof not less than 2×10¹³ atoms/cm³ and not more than 5×10¹⁵ atoms/cm³ bydoping nitrogen during the growth of the silicon ingot.

Preferably, a phosphorus concentration in a silicon wafer produced bythe above-described method may be not less than 4.3×10¹³ atoms/cm³ andnot more than 2.2×10¹⁴ atoms/cm³, and a resistivity of the wafer may beabout 20 Ω·cm to 100 Ω·cm.

In the above-described method for producing a silicon wafer for an IGBT,it is preferable to polish a surface of the wafer after the annealing ofthe wafer in the oxidizing atmosphere.

In the above-described method for producing a silicon wafer, by using asilicon ingot having a concentration of interstitial oxygen of not morethan 7×10¹⁷ atoms/cm³, it is possible to provide an excellent siliconwafer in which a reduction of the recombination lifetime caused by theformation of oxide precipitates (bulk micro defects: BMDs) during theproduction process of an IGBT, and fluctuation of resistivity caused bythe formation of oxygen donor are inhibited.

In addition, by irradiating neutrons to the silicon ingot, it ispossible to transform a predetermined amount of the silicon atoms intophosphorus atoms, thereby realizing a homogeneous doping of phosphorusin the silicon ingot from which wafers having a constant resistivity canbe obtained. By performing the annealing of the wafer under theoxidizing atmosphere at a temperature satisfying the above-describedformula (1), it is possible to form a silicon oxide layer on a surfaceof the silicon wafer. Being accompanied with the formation of thesilicon oxide layer, interstitial silicons are implanted to theinteriors of COPs occurring within the silicon wafer, thereby completelyfilling the COPs, and as a result, eliminate the COPs.

In silicon wafers obtained by the above-described method, COP defectsare scarcely exist in an interior of each wafer. In addition, variationof resistivity is reduced within a surface of a silicon wafer, and amongthe different wafers sliced from the same ingot. In the productionprocess of an IGBT, generation of BMDs (bulk micro defects) andfluctuation of resistivity are scarcely caused. Therefore, it ispossible to use a wafer of the invention as a suitable substrate for aninsulated gate bipolar transistor (IGBT).

In addition, since a polysilicon layer or a strained layer is formed ona surface of the wafer as a gettering layer, heavy metals contaminatingthe wafer can be sufficiently removed.

In the above described method for producing a silicon wafer, by dopingnitrogen in a silicon ingot, it is possible to remarkably reduce sizesof COPs in the infot. By annealing the silicon ingot under theabove-described conditions of heat treatment, it is possible tocompletely eliminate the COPs. In addition, by the doping of nitrogen inthe silicon crystal, it is also possible to inhibit the generation ofslip dislocations.

In the above-described method for producing a silicon wafer, bypolishing the wafer after the annealing of the wafer under the oxidizingatmosphere, it is possible to remove COPs remaining in the vicinity ofthe wafer surface after the annealing of the wafer under the oxidizingatmosphere. By removal of COPs in the vicinity of the wafer surface, itis possible to enhance the reliability of the gate oxide film.

A silicon wafer of the invention is a silicon wafer which is obtained byany one of the above-described methods for producing a silicon wafer.

A silicon wafer of the invention may be a silicon wafer which is slicedfrom a silicon ingot, the silicon wafer comprising:

a concentration of interstitial oxygen [Oi] of not more than 7×10¹⁷atoms/cm³;

being doped with phosphorus;

a polysilicon layer or a strained layer formed on one side of thesilicon wafer;

a density of COPs within the wafer of not more than 1×10⁵/cm³; and

a density of crystal defects of not more than 5×10⁷ /cm³ after beingsubjected to a double-step heat treatment including a heat treatment ofthe wafer for 4 hours at 800° C., and a heat treatment of the wafer for16 hours at 1000° C. It is preferable that density of COPs be not morethan 4.4×10⁴/cm³. It is more preferable that density of COPs be not morethan 1.0×10⁴/cm³.

In the above-described silicon wafer for an IGBT, it is preferable thatthe concentration of nitrogen be not less than 2×10¹³ atoms/cm³ and notmore than 5×10¹⁵ atoms/cm³.

In the above-described silicon wafer for an IGBT, it is preferable thatthe concentration of phosphorus in the wafer be not less than 4.3×10¹³atoms/cm³ and not more than 2.2×10⁴ atoms/cm³. Preferably, a resistivityof the silicon wafer is about 20 Ω·cm to 100 Ω·cm

The above-described silicon wafer has been produced by any one of theabove-described methods for producing a silicon wafer. Therefore, COPdefects scarcely exist in the interior of the wafer, and variation ofresistivity is reduced within the surface of a silicon wafer, and amongdifferent wafers sliced from the same ingot. In addition, in the siliconwafer, generations of BMDs and oxygen donors are scarcely caused duringa production process of an IGBT. Therefore, insulation breakdown causedby COPs, deterioration of recombination lifetime caused by generation ofBMDs, and fluctuation of resistivity caused by oxygen donors do notoccur in the silicon wafer. Therefore, the wafer of the invention can besuitably applied as a substrate of an insulated gate bipolar transistor(IGBT).

In addition, the above-described silicon wafer is a phosphorus-dopedwafer of high resistivity, the wafer is suitably applied as a substrateof an IGBT. In addition, since one side of the surface of the siliconwafer is provided with a polysilicon layer or a strained layer as agettering layer, it is possible to remove contamination of heavy metalsduring the production process of the IGBT.

In addition, the above-described silicon wafer is obtained from acrystal having a concentration of nitrogen of not less than 2×10¹³atoms/cm³ and not more than 5×10¹⁵ atoms/cm³, and small sized COPs.Therefore, COPs are almost completely eliminated in the wafer, and thewafer can be suitably used as a substrate for an IGBT. In addition, bydoping of nitrogen, generation of slip dislocations can be inhibited,and the quality of the wafer can be enhanced.

In addition, the production process of an IGBT includes various heatingsteps by which a silicon wafer for the IGBT is unavoidably heated. Inthe silicon wafer of the invention, even after being influenced byvarious thermal histories corresponding to the heating steps in theproduction process of an IGBT, resistivity is not changed and therecombination lifetime is not reduced in the silicon wafer. Therefore,the wafer can be suitably used as a substrate of various devicesexemplified by an IGBT.

As explained above, in the method for producing a silicon wafer of theinvention, by annealing a CZ silicon ingot having an extremely lowconcentration of interstitial oxygen under an oxidizing atmosphere, itis possible to eliminate COPs in the wafer. In addition by transforminga predetermined amount of the silicon atoms into phosphorus atoms byirradiating neutrons to the silicon ingot, it is possible to obtain asilicon wafer having a constant resistivity.

A silicon wafer obtained by the above-described method scarcely includesCOPs, and has a constant resistivity. Therefore, the wafer of theinvention can be suitably used as a substrate of an insulated gatebipolar transistor (IGBT).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between the concentration ofinterstitial oxygen in a wafer, annealing temperature, and asuccessiveness in extinction of COPs in the wafer.

FIG. 2A and FIG. 2B are photographs taken by X-ray topography of siliconwafers according to example 1.

FIGS. 3A to 3D are photographs taken by x-ray topography of siliconwafers according to Example 2 and Example 3.

FIG. 4A and FIG. 4B are distribution maps indicating the distribution ofthe recombination lifetime in a wafer before and after heat treatment ofthe wafer according to Example 2.

FIG. 5A and FIG. 5B are distribution maps indicating the distribution ofthe recombination lifetime in a wafer before and after heat treatment ofthe wafer according to Example 3.

FIG. 6A and FIG. 6B are distribution maps indicating the distribution ofthe resistivity in wafers according to Example 2.

FIG. 7A and FIG. 7B are distribution maps indicating the distribution ofthe resistivity in wafers according to Example 3.

DETAILED DESCRIPTION OF THE INVENTION

As an embodiment of the invention, a method for producing a siliconwafer is explained in the following.

A method for producing a silicon wafer of the invention comprises thefollowing steps. A silicon ingot is grown by the Czochralski method (CZmethod) to have an interstitial oxygen concentration [Oi] of not morethan 7.0×10¹⁷ atoms/cm³. Next, after doping phosphorus in the siliconingot by irradiating a neutron beam to the ingot, wafers are sliced fromthe ingot. Next, each wafer is annealed in an oxidizing atmospherecontaining at least oxygen at a temperature of T(° C.) that satisfies aformula,[Oi]≦2.123×10²¹ exp(−1.035/k(T+273))  (1).Then, a polysilicon layer or a strained layer is formed on one side ofthe wafer.

In the above-described formula (1), [Oi] is measured by Fouriertransform infrared spectrometry based on ASTM F-121(1979); and k isBoltzman constant (8.617×10⁻⁵(eV/K)).

In the above-described method for producing a silicon wafer for an IGBT,during growth, it is preferable to form a silicon ingot having anitrogen concentration of not less than 2×10¹³ atoms/cm³ and not morethan 5×10¹⁵ atoms/cm³ by doping nitrogen during the growth of thesilicon ingot.

The nitrogen concentration may be analyzed secondary ion massspectrometry (SIMS). Using SIMS, it is difficult to measure a nitrogenconcentration in an order of 1×10¹³ atoms/cm³. In such a case, thenitrogen concentration may be calculated from an amount of dopant dopedin the formation process of an ingot and segregation coefficient ofnitrogen.

In addition, in the method for producing a silicon wafer of the presentembodiment, a strained layer may be formed on back side of the wafer bya sand blasting method or the like before annealing of the wafer underthe oxidizing atmosphere.

Each step is explained in the following.

A silicon ingot having an interstitial oxygen concentration [Oi] of notmore than 7.0×10¹⁷ atoms/cm³ may be produced by the Czochralski method(CZ method). Production of the silicon ingot by the CZ method isperformed in the following manner. Firstly, a block of polycrystallinesilicon is loaded in a quartz crucible of a pulling apparatus. A siliconmelt is formed by heating the block of polycrystalline silicon in anargon (Ar) atmosphere. Next, a seed crystal is immersed in the siliconmelt and is gradually pulled up while rotating the seed crystal and thecrucible, thereby a single crystal is grown under the seed crystal. Atthat time, as an example of growth conditions, a ratio V/G of a growthrate V (mm/minute) of the crystal and thermal gradient G (° C./mm)between the melting point and 1350° C. may be controlled to be in anorder of 0.22 to 0.27 during the growth of the crystal. As an example ofadditional conditions, it is possible to control the cycle of the quartzcrucible to be 0.05 to 0.5 rpm, pressure of the Ar atmosphere to be 30torr, and magnetic field strength to be 3500 Gauss. By controlling theinterstitial oxygen concentration [Oi] of the silicon ingot to be notmore than 7.0×10¹⁷ atoms/cm³, generation of oxygen donors during aproduction process of an IGBT can be inhibited. An interstitial oxygenconcentration [Oi] exceeding 7.0×10¹⁷ atoms/cm³ is not preferable,because at such a concentration, oxygen donors are generated during theproduction process of the IGBT, and change the properties of the IGBT.

By doping nitrogen in the silicon ingot, sizes of COPs are reduced, andthe COPs can be eliminated by annealing for a relatively short timeperiod. In addition, by the nitrogen doping, it is possible to cause aneffect of pinning a dislocation by nitrogen, and it is possible toinhibit the generation of slip dislocations during heat treatment at ahigh temperature. As a method for nitrogen doping, it is possible toapply any known method. For example, by melting a silicon wafer having anitride film along with a polycrystalline silicon raw material, it ispossible to grow a silicon ingot doped with nitrogen.

Next, the silicon ingot which has been produced by the above-describedmethod is subjected to neutron beam irradiation. By the neutron beamirradiation, a predetermined amount of the silicon atoms are transformedto phosphorus atoms, thereby the silicon ingot is homogeneously dopedwith phosphorus and has homogeneous resistivity. The silicon ingotincludes silicon 30(³⁰Si) as an isotope of silicon. By neutronirradiation, ³⁰Si may be converted to ³¹Si which transforms to stableisotope phosphorus 31 (³¹P) by nuclear transformation. When an n typesilicon single crystal is doped with phosphorus by a method of addingphosphorus to a silicon melt from which the silicon single crystal ispulled, the resistivity changes along a pulling direction of the ingot,and causes changes of the resistivity of the ingot. Therefore, in theinvention, it is necessary to use a neutron irradiation method by whichthe concentration of dopant in the silicon ingot can be homogeneouslydistributed. For example, conditions for the neutron beam irradiationcan be controlled such that at a position at which the flux of neutronbeam is 3.0×10¹² neutrons/cm²/s, an ingot is irradiated with theneutrons for 80 hours while being rotated at a cycle of 2 rpm.

Thus, after the neutron irradiation, a silicon ingot acquires aresistivity of 48 Ω·cm to 52 Ω·cm.

Next, wafers are sliced from the silicon ingot. Where necessary, eachwafer is subjected to lapping, etching or the like. Subsequently, wherenecessary, a strained layer is formed on the silicon wafer by a sandblasting method or the like.

Next, the wafer is annealed in an oxidizing atmosphere. The atmospherefor the annealing is not limited provided that oxygen is contained inthe atmosphere. For example, a mixed gas of oxygen and an other gas suchas nitrogen or argon may be used. However, in order to shorten the timeperiod which is required for eliminating COPs, it is preferable to usean oxygen atmosphere and a mixed atmosphere of oxygen and water vapor.

The annealing should be performed at a temperature T(° C.) whichsatisfies the formula,[Oi]≦2.123×10²¹ exp(−1.035/k(T+273))  (1).

In the above-described formula (1), [Oi] is an interstitial oxygenconcentration in the silicon wafer, measured by Fourier transforminfrared spectrometry based on ASTM F-121(1979); and k is Boltzmanconstant (8.617×10⁻⁵(eV/K)). By annealing the wafer under conditions offormula (1), it is possible to eliminate COPs in the wafer almostcompletely. More specifically, where the interstitial oxygenconcentration [Oi] is 7.0×10¹⁷ atoms/cm³, the annealing temperatureshould be not lower than 1230° C. and not higher than the melting pointof silicon. At the same temperature, as COPs have large sizes, a longannealing time is required. Therefore, the time period for annealingcannot be fixed to a limited value. For example, in order to eliminateCOPs of 0.17 μm in size by annealing at a temperature of 1150° C., anannealing time of 2 hours is required. By doping nitrogen at the time ofpulling the crystal, COP sizes are reduced, and the time period forannealing can be shortened.

It is possible to use different atmospheres for the heating step andannealing step at an annealing temperature. For example, it is possibleto use mixed gas atmosphere containing oxygen for heating a crystal upto an annealing temperature, and a pure oxygen atmosphere for annealingof the crystal at the annealing temperature.

Next, a polysilicon layer can be formed on one side of the wafer. Sincethe silicon wafer of the present embodiment has an extremely low oxygenconcentration, it is impossible to expect a gettering effect by oxygen.Therefore, after the annealing of the wafer in an oxidizing atmosphere,one side of the wafer must be provided with a polysilicon layer as agetteing layer in order to remove the contamination with heavy metalscaused during the production process of an IGBT. A thickness of thepolysilicon layer may be not less than 0.5 μm and not more than 2 μm. Apolysilicon layer of not less than 0.5 μm in thickness can exhibit asufficient gettering effect. A thickness of not more than 2 μm issufficient for inhibiting warpage of the wafer.

A silicon wafer produced by the above-described method comprises aninterstitial oxygen concentration [Oi] of not more than 7×10¹⁷atoms/cm³, and is doped with phosphorus, and is provided with apolysilicon layer or a strained layer on one side of the wafer. Inaddition, an internal portion of the wafer scarcely contains COPs.Preferably, the nitrogen concentration of the wafer is not less than2×10¹³ atoms/cm³ and not more than 5×10¹⁵ atoms/cm³. More preferably,the phosphorus concentration in the wafer is not less than 4.3×10¹³atoms/cm³ and not more than 2.2×10¹⁴ atoms/cm³, and the resistivity isabout 20 Ω·cm to 100 Ω·cm. Such a silicon wafer can be suitably used asa substrate for an IGBT.

While the silicon wafer of the present embodiment is suitably used as asubstrate for an IGBT, the production process of the IGBT includesvarious heating steps, by which the silicon wafer is unavoidably heated.In the silicon wafer of the present embodiment, even after beingsubjected to various thermal histories corresponding to the heatingsteps in the production process of an IGBT, the resistivity is notchanged and the recombination lifetime is not reduced in the siliconwafer. Therefore, the wafer can be suitably used as a substrate ofvarious devices exemplified by an IGBT. Specifically, even after beingsubjected to a double-step heat treatment including a heat treatment at800° C. for 4 hours and heat treatment at 1000° C. for 16 hours, thepercentage reduction of the recombination lifetime before and after theheat treatment is within the range of 20%. With a wafer having aresistivity of 50 Ω·cm, even after heat treatment of the wafer at atemperature of 450° C. for 1 hour, the reduction of the resistivity iscontrolled to be about 8%.

EXAMPLES

In the following, the present invention is explained in detail based onExamples. Example 1

Firstly, silicon ingots of various interstitial oxygen concentrationswere produced by the CZ method. Specifically, each single crystallinesilicon ingot was prepared by the following steps. Firstly, a block ofpolycrystalline silicon was loaded in a quartz crucible, and a siliconmelt was formed by heating the block of polycrystalline silicon in an Aratmosphere. Next, a seed crystal was immersed in the silicon melt andwas gradually pulled up while rotating the seed crystal and thecrucible, thereby a single crystal was grown under the seed crystal. Atthat time, as an example of growth conditions, a ratio V/G of a growthrate V (mm/minute) of the crystal and thermal gradient G (° C./mm)between the melting point and 1350° C. was controlled to be about 0.27.The interstitial silicon concentration was controlled by controllingrotation cycle of the quartz crucible and the pressure of the argonatmosphere. Interstitial oxygen concentration may be reduced by loweringthe cycle of the quartz crucible, or by reducing a pressure of the argonatmosphere. In addition, by using a MCZ method (magnetic field appliedCzochralski method), it is easy to produce a silicon ingot of low oxygenconcentration. Thus, a plurality of silicon ingots having aninterstitial oxygen concentration within a range of 3×10¹⁷ atoms/cm³ to6×10¹⁷ atoms/cm³ were produced. Wafers were sliced from the ingots.After performing etching of the wafers in order to remove work strain,COP densities in the wafers were measured. Each wafer showed a COPdensity within a range of 3×10⁶/cm³ to 6×10⁶/cm³. In the measurement ofthe COP densities, an OPP (Optical Precipitate Profiler) of AccentOptical Technologies was used.

Next, silicon ingots which had been prepared under the same conditionsas the above-described growth conditions were doped with phosphorus byneutron beam irradiation to the ingots. Each ingot was irradiated with aneutron beam having a flux of 3.0×10¹² neutrons/cm²/s for 80 hours.After that, wafers were sliced from the silicon ingots. The slicedwafers were subjected to surface treatment including lapping andetching.

Next, the wafers were introduced in an annealing apparatus, and weresubjected to heat treatment in an oxidizing atmosphere. The annealingtemperature T (° C.) was controlled within a range of 1050° C. to 1200°C. The atmosphere for the annealing was controlled to be an oxygenatmosphere comprising 100% of oxygen. Thus, silicon wafers of 150 mm indiameter having various interstitial oxygen concentrations and beingtreated with various annealing temperatures were obtained.

COP densities in the wafers were measured by the OPP. In order to avoidthe influence of surface irregularities of one side and the other sideof a wafer, both sides of each wafer were polished before the OPPmeasurement, and the lower limit of the detection size was set at 30 nm.Where the density of defects was not more than 4.4×10⁴/cm³, it wasconsidered that COPs had been eliminated.

The interstitial oxygen concentration was measured by the Fouriertransform infrared spectrometry based on a standard of ASTM F-121(1979).The results are shown in FIG. 1. In FIG. 1 the vertical axis showsinterstitial oxygen concentrations, and the horizontal axis showsannealing temperatures. Open circles indicate wafers in which COPs areregarded to have been eliminated, and crucial marks indicate wafers inwhich COPs are regarded to remain.

As shown in FIG. 1, where a dashed line shows boundary conditions, COPsremained in wafers having an oxygen concentration higher than theboundary, and COPs were eliminated in wafers having an oxygenconcentration lower than the boundary. The following formula wasobtained to approximate the boundary conditions,[Oi]=2.123×10²¹ exp(−1.035/k(T+273))  (2).

Therefore, it was determined that suitable conditions for the annealingtemperature were expressed by the formula,[Oi]≦2.123×10²¹ exp(−1.035/k(T+273))  (3).

Next, wafers in which COPs were regarded as having been eliminated weresubjected to X-ray topography to examine presence of slip dislocations.In the examination for the presence of slip dislocations on wafershaving an oxygen concentration of 5.5×10¹⁷ atoms/cm³ and being heattreated at 1200° C., as shown in FIG. 2A and FIG. 2B, it was confirmedthat slip dislocations were not observed in the wafers.

Next, yields of gate oxide integrity (GOI) in the wafers were examinedusing the conditions of gate oxide thickness of 25 nm, electrode area of8 mm², and criteria for electric field strength of 11 MV/cm. As aresults, the yields of gate oxide integrity were 100%.

Example 2

Silicon ingots having various interstitial oxygen concentrations wereproduced by the CZ method. Those ingots were doped with nitrogen.Specifically, each single crystalline silicon ingot was prepared by thefollowing steps. Firstly, a block of polycrystalline silicon was loadedin a quartz crucible, and a silicon melt was formed by heating the blockof polycrystalline silicon in an Ar atmosphere. As a dopant of nitrogen,a silicon wafer having a nitride film was loaded to the silicon melt.Next, a seed crystal was immersed in the silicon melt and was graduallypulled up while rotating the seed crystal and the crucible, thereby asingle crystal was grown under the seed crystal. At that time, thepulling rate of the crystal was controlled to be about 1.2 mm/minute.Thus, silicon ingots having an interstitial oxygen concentration of3.5×10¹⁷ atoms/cm³ and nitrogen concentration of 2.5×10¹⁴ atoms/cm³ wereproduced.

COP density of the silicon ingots was measured to be 2.0×10⁷/cm³.

Next, in a same manner as Example 1, the silicon ingots were doped withphosphorus by neutron beam irradiation. After that, wafers were slicedfrom the ingots. The sliced wafers were subjected to surface treatmentsincluding lapping and etching or the like.

Next, the wafers were introduced in an annealing apparatus, and weresubjected to annealing in an oxidizing atmosphere. The annealingcondition was controlled to comprise heating up to 1100° C. at a heatingrate of 5° C./minute, and heating up to 1050° C. at a heating rate of 1°C./minute, and annealing the wafers for 3.5 hours at an annealingtemperature of 1150° C., and cooling to 900° C. at a cooling rate of 2°C./minute. In the above-described conditions, the annealing temperaturesatisfied the above described formula (3). During the heating theatmosphere was controlled to be a nitrogen atmosphere containing 3% ofoxygen by volume. After reaching the annealing temperature, theatmosphere was changed to an oxygen atmosphere comprising 100% ofoxygen. The time period between inserting the wafers into the furnaceand extracting the wafers from the furnace was controlled to be 14hours. Thus, silicon wafers of Example 2 of 200 mm in diameter wereobtained.

Example 3

Silicon wafers of Example 3 of 200 mm in diameter were prepared by thesame manner as Example 2 except that a silicon melt was not loaded witha silicon wafer having a nitride film. The silicon wafers of Example 3had the same interstitial oxygen. concentration as that of the siliconwafers of Example 2.

Evaluation of Example 2 and Example 3

COP densities of silicon wafers of Example 2 and Example 3 were measuredby the same method and under the same conditions as those of Example 1.COP densities of the wafers of Example 2 and Example 3 were not morethan 4.4×10⁴/cm³. Therefore, it was confirmed that COPs were eliminatedby the annealing in an oxidizing atmosphere.

Next, the occurrence of slip dislocations in wafers of Example 2 andExample 3 were examined by X-ray topography. FIGS. 3A to 3D show imagesof X-ray topography. As shown in FIGS. 3A to 3D, a slip dislocation wasgenerated in lower right of the wafer of each example. In thenitrogen-doped wafers of Example 2, the length of the slip dislocationwas within a range of 0.3 to 0.8 cm. While wafers of Example 3 not dopedwith nitrogen showed a slip length within a range of 0.8 to 1.5 cm. Thatis, slip lengths in wafers of Example 3 were slightly longer than thoseof Example 2. Compared to Example 3, shortening of slip length in wafersof Example 2 was considered to be an effect of nitrogen-doping.

Next, yields of gate oxide integrity (GOI) in the wafers were examinedusing conditions of a gate oxide thickness of 25 nm, electrode area of 8mm², and criteria for electric field strength of 11 MV/cm. As a results,the yields of gate oxide integrity were not less than 95%.

Next, the recombination lifetime in wafers of Example 2 and Example 3was measured. In the measurement of recombination lifetime, wafers ofexample 2 and Example 3, and wafers obtained by performing predeterminedheat treatment on the wafers of Example 2 and Example 3 were subjectedto the measurement. The measurement of recombination lifetime wasperformed by a μ-PCD method. In addition, as the heat treatmentconditions, heat treatment conditions analogous to those for aproduction process of an IGBT, in which the lowest temperature was 350°C. and highest temperature was 1150° C., were used. FIG. 4A and FIG. 4Bshow a distribution of the recombination lifetime in a silicon wafer ofExample 2 before and after the heat treatment. FIG. 5A and FIG. 5B showa distribution of the recombination lifetime in a silicon wafer ofExample 3 before and after the heat treatment. Distribution ranges ofthe recombination lifetime and average value of the recombinationlifetime in wafers of Example 2 and Example 3 are listed in Table 1.

TABLE 1 Recombination lifetime (μsec) Average Distribution Heattreatment value range Example 2 Before heat treatment 2242 1362 to 3021nitrogen-doped After heat treatment 1869  271 to 2688 Example 3 Beforeheat treatment 2186 1792 to 2980 not doped with After heat treatment1832 1265 to 2463 nitrogen

As shown in FIG. 4A, FIG. 4B, FIG. 5A, FIG. 5B and Table 1, therecombination lifetime is slightly shortened by the heat treatment. Thelevel of the recombination lifetime is a sufficient value as a substrateof an IGBT.

Before formation of an IGBT, a wafer is subjected to various heattreatments. The above-described conditions for heat treatment weredetermined in order to simulate the thermal history, to which a waferwas subjected before the production of the IGBT. A wafer treated withthe above-described heat treatment can be considered as a wafersubjected to the same thermal history as an IGBT.

As described above, even after being worked to an IGBT, wafers ofExample 2 and Example 3 had a sufficient recombination lifetime.Therefore, those wafers were suitable as substrates of devices such asan IGBT or the like.

Next, the resistivity of the wafers of Example 2 and Example 3 wasmeasured before and after the heat treatment of the wafer. The resultsfor Example 2 are shown in FIG. 6A and FIG. 6B. The results for Example3 are shown in FIG. 7A and FIG. 7B. FIG. 6A and FIG. 6B indicate arelationship between resistivity and points of measurement. FIG. 6B andFIG. 7B indicate points of measurement on each wafer. In each wafer,measurements of the resistivity were performed at 17 points comprising acenter point (1), eight points (2 to 9) aligned on a virtual circlehaving a radius of ½ of a wafer radius, and eight points (10 to 17) on avirtual circle having a radius which is smaller than the wafer radius by5 mm.

As it is shown in FIG. 6A, FIG. 6B, FIG. 7A, FIG. 7B, the resistivity ofthe wafers scarcely changed before and after the heat treatment. Inaddition, it was also confirmed that nitrogen-doped wafers of Example 2and wafers of Example 3 not doped with nitrogen showed hardly anydifference in resistivity. Those results indicated that the generationof thermal donors and N—O donors (complex of nitrogen and oxygen) wasinhibited because of a sufficiently low oxygen concentration in thecrystal. Comparing the resistivity from the center point (1), eightpoints (2 to 9) on the circle having a radius of ½ of a wafer radius,and eight points (10 to 17) on a peripheral portion of the wafer,variations in a range of 2 to 3 Ω·cm were observed in resistivity.Considering that the resistivity of the wafer is in a range of 60 to 64Ω·cm, the variation is not a sufficient level to cause problems onquality of the wafer. Therefore, the wafers were suitable as substrateof an IGBT.

Example 4

By controlling the cycle of a quartz crucible and a pressure of argonatmosphere, 4 silicon ingots respectively having an oxygen concentrationof 5.6×10¹⁷ atoms/cm³ (sample No. 1), 7.0×10¹⁷ atoms/cm³ (sample No. 2),8.1×10¹⁷ atoms/cm³ (sample No. 3), 9.5×10¹⁷ atoms/cm³ (sample No. 4)were pulled. Each ingot was doped with nitrogen of 3×10¹⁴ atoms/cm³. Byneutron irradiation on the ingots, each ingot acquired a resistivity of50 Ω·cm. Wafers having a diameter of 150 mm were sliced from thoseingots. The wafers were subjected to annealing in an oxygen atmospherecomprising 100% of oxygen for 1 hour at 1200° C. After the annealing,the surface of the wafer was mirror polished. Thus, sample wafers of No.1 to No. 4 were prepared.

Under the same conditions as Example 1, evaluation of the yield of gateoxide integrity (GOI) was performed on each wafer. In addition, eachwafer was subjected to a double-step heat treatment including a heattreatment at 800° C. for 4 hours and a heat treatment at 1000° C. for 16hours. The crystal defect density after the heat treatment, and changeof the recombination lifetime before and after the heat treatment wereexamined for each wafer. In addition, each sample wafer was subjected toa heat treatment at 450° C. for 1 hour, and change in the resistivitybefore and after the heat treatment was examined. The double-step heattreatment comprising a heat treatment at 800° C. for 4 hours and a heattreatment at 1000° C. for 16 hours is a typical heat treatment which isembodied to evaluate an oxide precipitation property of a wafer. Theheat treatment at 450° C. for 1 hour is a heat treatment for simulatinga sintering treatment of aluminum circuits, performed in a latter stageof the production process of an IGBT. Allowance of resistivity in asubstrate for the IGBT is typically in a range of ±8%, and the targetvalue of resistivity in the present embodiment was 50 Ω·cm. Therefore,the allowable range of resistivity was in a range of 46 to 54 Ω·cm. Theresults of evaluation are shown in Table 2.

TABLE 2 Sample No. No. 1 No. 2 No. 3 No. 4 [Oi] (atoms/cm³) 5.6 × 10¹⁷7.0 × 10¹⁷ 8.1 × 10¹⁷ 9.5 × 10¹⁷ GOI yield (%) 99.1 27.0 26.6 24.9Density of crystal defects (/cm³) not more than 5.0 × 10⁷  2.2 × 10⁸ 7.2 × 10⁸  4.4 × 10⁴ Recombination Before heat treatment 2095 1986 20241956 lifetime After heat treatment 2056 1932 97 56 (μsec) ResistivityBefore heat treatment 50.1 50.2 49.9 50.3 (Ω · cm) After heat treatment48.0 46.4 25.8 23.8

Regarding the yield of GOI, sample wafer No. 1 shows a yield of nearly100% since only in sample wafer No. 1 did the oxygen concentrationsatisfy the conditions of the formula (1) at the annealing temperatureof 1200° C. In order to eliminate COPs of respective sample wafers ofNo. 2, No. 3, and No. 4, annealing of the wafer at a temperature of notlower than 1226° C., not lower than 1253° C., and not lower than 1285°C. was required respectively.

Next, crystal defect densities measured by OPP and the recombinationlifetime after a double-step heat treatment were measured for each waferat 800° C. for 4 hours and at 1000° C. for 16 hours. In sample No. 1,crystal defects were not detected, since COPs were eliminated by oxygenannealing, and oxide precipitates (BMDs) were not generated by thedouble-step heat treatment. Since BMDs did not occur in sample wafer No.1, the recombination lifetime was not reduced in the sample wafer No. 1.In sample No. 2, defects of 5.0×10¹⁷/cm³ were detected. Judging from thedensity of the defects, most of the defects which were detected wereconsidered to be COPs. Therefore, the recombination lifetime was hardlyreduced in sample No. 2.

In samples No. 3 and No. 4, reduction of the recombination lifetimeoccurred because of the generation of BMDs during the double-step heattreatment.

Regarding the resistivity, the resistivity of the sample wafer decreasesin an order of No. 1, No. 2, No. 3 and No. 4, since the concentration ofoxygen donors generated by the heat treatment at 450° C. increases withincreasing oxygen concentration in the wafer. Therefore, sample wafersof No. 1 and No. 2 were outside the allowable range of 50 Ω·cm±8% (46 to54 Ω·cm).

By raising the annealing temperature, COPs could be eliminated. However,in order to inhibit the reduction of recombination lifetime caused bythe generation of BMDs, and to inhibit the reduction of the resistivity,it is necessary to control the oxygen concentration of the wafer to benot more than 7×10¹⁷ atoms/cm³.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1. A silicon wafer for an IGBT, which is sliced from a silicon ingotproduced by the Czocralski method, the silicon wafer comprising: aconcentration of interstitial oxygen [Oi] of not more than 7×10¹⁷atoms/cm³; being doped with phosphorus; a polysilicon layer or astrained layer formed on one side of the silicon wafer; a density ofCOPs within the wafer of not more than 4.4×10⁴/cm³; and a density ofcrystal defects of not more than 5×10⁷/cm³ after being subjected to adouble-step heat treatment including a heat treatment of the wafer for 4hours at 800° C., and a heat treatment of the wafer for 16 hours at1000° C.
 2. A silicon wafer for an IGBT according to claim 1, wherein aconcentration of nitrogen in the wafer is not less than 2×10¹³ atoms/cm3and not more than 5×10^(‥)atoms/cm³.